This invention relates generally to broadband communications systems, such as a hybrid/fiber coaxial (HFC) cable television system, and more specifically to a communications network allowing a method for transmitting reverse optical signals in the broadband communications system by sub-sampling a limited bandwidth within the reverse bandwidth.
A conventional broadband communications system 100, such as a two-way hybrid/fiber coaxial (HFC) communications system, that carries analog and optical signals is depicted in FIG. 1. The communications system 100 includes headend equipment 105 for generating forward signals that are transmitted in the forward, or downstream, direction along a first communication medium, such as a fiber optic cable 110. Coupled to the headend 105 are optical nodes 115 that convert the optical signals to radio frequency (RF) signals. The RF signals are further transmitted along a second communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more express amplifiers 130 positioned along the communication medium. Tap amplifiers 135 are typically positioned along the end of the express lines to tap off the RF signals, for example, in three different directions. Taps 140 then further split off portions of the forward signals for provision to subscriber equipment 145, such as set-top terminals, computers, modems, and televisions. It will be appreciated that there are typically several different fiber links connecting the headend 105 with several additional nodes 115, amplifiers 130, 135, and subscriber equipment 145.
In a two-way system, the subscriber equipment 145 can also generate reverse electrical signals that are transmitted in the reverse, or upstream, direction to the headend equipment 105. Any one or more of the distribution amplifiers 130, 135 may amplify such reverse signals. The signals are then converted to optical signals by the optical node 115 before being provided to the headend equipment 105.
Conventionally, an analog communications system transmits and receives the forward and reverse signals in the analog domain. FIG. 2 is a block diagram of an example of an optical link network that includes a headend and optical nodes in further detail. This example is suitable for use in the analog broadband communications system 200. A headend 205 generates and transmits optical signals via optical transmitters 210a-n downstream through their respective fiber links 215a-n. It will be appreciated that there are a plurality of optical transmitters 210a-n transmitting optical signals to a plurality of nodes 220a-n, where, depending upon the network design, each node 220 typically services a different portion of the system. Within the nodes 220a-n, an optical receiver 230a-n, among other operations, converts the optical signals to electrical signals. A diplex filter 235a-n then isolates the forward electrical signals from the reverse path and provides the electrical signals to coaxial cable 240a-n for delivery to the subscriber equipment 225a-n. 
In the reverse path, electrical signals emanating from subscriber equipment 225a-n are transmitted upstream via the coaxial cable 240a-n to the node 220a-n. The diplex filter 235a-n isolates the reverse signals from the forward path and provides the signals to an optical transmitter 245a-n for conversion of the electrical signals to optical signals. The optical signals are then transmitted upstream, via an optical fiber 248a-n, to an optical receiver 250a-n that may be located within the headend 205, where the information is processed.
If additional subscriber homes are added to the network 200, it may be necessary to add an additional node 220 that includes separate links for the forward and reverse path to address the additional subscriber equipment within the homes. Additionally, if the operator chooses to optimize the network 200 to accommodate an increase in the amount of reverse signals being transmitted by one optical transmitter, an operator can accomplish this by decreasing the number of subscriber homes that a node 220 services. For example, an operator can reduce an existing network that includes 2000 subscriber homes per node to 500 subscriber homes per node, and add three additional nodes to the network. It can easily be understood that increasing the size or optimizing the network requires a significant amount of equipment and fiber.
It will be appreciated that separate reverse fiber paths, or links 248a-n, are required for each node because reverse optical signals cannot be combined like reverse electrical signals. More specifically, those skilled in the art will appreciate that when the light from multiple optical transmitter outputs, where each output has a specific wavelength, is applied simultaneously to an optical receiver, intermodulation distortion results. If the differences between these received wavelengths are sufficiently small, the intermodulation distortion produced in the optical receiver will obscure the desired electrical signals, which are, for example, signals within the range from 5 MHz to 42 MHz, at the output of the optical receiver. The drift in wavelength encountered in conventional optical transmitters makes this condition likely to happen.
Recently, new broadband applications, such as interactive multimedia, Internet access, and telephony, are increasing the number of reverse signals within the reverse bandwidth. As a result, network operators are redesigning networks to effectively increase the total reverse signal carrying capacity, for example, by digitizing the reverse analog signals and, therefore, allowing more digital signals to be transmitted within the existing reverse bandwidth. More specifically, a simplified digital reverse communications path that can be used in a broadband communications system to digitize analog signals is depicted in FIG. 3. Digitizing the optical signals as shown in FIG. 3 allows the operator to increase the reverse signal carrying capacity that is demanded by the growing number of customers and broadband applications.
Briefly, a plurality of digital transmitters 305a-n, each including an analog-to-digital (A/D) converter 308a-n, receives analog electrical signals from a number of pieces of connected subscriber equipment and converts the analog electrical signals to digital optical signals. Linked, via fiber optic cable 309a-n, to each digital transmitter 305a-n is a digital receiver 310a-n that includes a digital-to-analog (D/A) converter 315a-n and which is located further upstream in the network 300. The D/A converter 315a-n converts the received digitized optical signals back to analog electrical signals for delivery to the headend and further processing. An example of a similar digital reverse path is discussed further in commonly assigned, copending patent application Ser. No. 09/102,344, filed Jun. 22, 1998, in the name of xe2x80x9cDigital Optical Transmitterxe2x80x9d, the disclosure of which is incorporated herein by reference.
To address the new broadband applications and interactive services, system operators are focusing on efforts to drive fiber deeper into neighborhoods and directly into subscribers"" homes. The operators need a cost-effective way to add more signals within the existing bandwidth and make two-way capable networks truly two-way active. FIG. 4 is a block diagram of one example illustrating a combination network including an HFC analog network and an overlayed digital network for increasing the signal carrying capacity for broadband applications. In this example, the network 400 has a portion of the system 402 that carries the traditional forward and reverse signals, which can be analog and/or digital signals, using, for example, existing communications equipment, such as analog and digital headend equipment 405 to generate and process analog signals. These analog signals, such as cable television signals, cable modem signals, and e-mail signals can be digitized and routed through an optical fiber link 410 to an optical node 415. The optical node 415 converts the optical signals back to electrical signals. The node 415 then transmits the signals through coaxial cable 420a-n in, for example, several directions. Splitters 425a-n, such as amplifiers or taps, then split the signals further for delivery to subscriber equipment 430a-n via coaxial cable.
A second portion of the network 435 may be overlayed with the first portion of the network 402 to deploy digital interactive services, such as, telephony, Ethernet, and other high-speed data services. Digital headend equipment 440 generates and processes these signals with separate digital equipment that includes, for example, high-speed switches and routers. These digital signals are transmitted through a network of fiber optic cable 445 and secondary hubs 450 to transport the digital signals long distances. A hub 455, which can be, for example, a hub that includes a router, routes the digital signals to the intended subscriber equipment 430a-n, typically via fiber optic cable. Reverse signals from the network 400 are transmitted upstream, via either the coaxial cable or the fiber optic cable, through the different networks 402, 435 back to the respective headend equipment 405, 440 for processing. This network 400 is a network that enables operators to use existing equipment they may already have deployed in the HFC network, such as set-tops, amplifiers, nodes, and taps, in addition to adding a digital network that increases their ability to process broadband applications. Additionally, they may be able to offer services that they were not able to provide with just the existing HFC analog network 402.
Digitizing the reverse path as shown in FIG. 3 and deploying advanced technology and systems as shown in FIG. 4 to offer expanded services, such as high-speed data and two-way interactive applications, can be expensive, however. Most network operators are not ready to invest in the required capital costs to change their existing networks to digital networks. Additionally, an HFC analog network 402 is still required to transmit the analog cable television channels; for example, a subscriber""s local channels are typically in an analog format. Typically, network operators that have been operating for a substantial length of time do not have the digital equipment, such as routers, switches, digital transmitters, and digital receivers, required to digitize or route the reverse signals. In order to accomplish this, the operators would have to substantially upgrade their system to include the digital equipment and may also have to lay extensive routes of fiber. Again, the majority of operators have historically transmitted and received analog signals over an analog HFC system; therefore, due to the expensive undertaking of sending digital reverse signals, most operators would like an intermediate step to enable an efficient, low-cost delivery of reverse signals over their existing HFC system.
Thus, in summary, what is needed is a network allowing a method of transmitting the traditional analog reverse signals throughout an existing HFC network that also simplifies the transition from the HFC network to a digital network. This network should not substantially increase the costs of required equipment giving the operators time to transition between the different networks.